Commodity history has been the boom genre of non-fiction over the past decade. There have been histories written of the potato (twice), sugar (twice), coffee (thrice), gold (twice), jade, porcelain (splendidly, once), various dyes (mauve, indigo), nutmeg and sundry other spices. At their worst, these commodity histories are complacent annals of consumption; at their best, they up-end our ideas of history's motive forces.

The American food writer and journalist Mark Kurlansky gained fame with the fine Cod: The Biography of a Fish (1997), and in Cod are to be found the grains of Salt, his huge and hugely impressive new book. It was in Cod that Kurlansky first wrote of the 'perfect marriage' between salt and fish, and of how salt could shape history, citing how the Basques' discovery of the preservative properties of salt allowed them to sail further afield even than the Vikings.

Kurlansky's new book is subtitled A World History, and it is one of the few commodity histories to merit such a moniker, because salt isn't just a seasoning, it's a life substance, vital to the proper functioning of the human body. For as long as there have been humans, they've had to find or create salt to live. The history of salt is the history of humanity.

Second only to salt's physiological importance has been its use as a food preservative. It was the only way of decelerating putrefaction until technological advances in the twentieth century, notably the fast-freezing method pioneered by American eccentric Clarence Birdseye, but it remains intrinsic to our lives. Contemporary industry pundits claim salt has more than 14,000 uses.

Every piece of evidence in this book is arranged to point to salt as an agency of enormous power. It has determined the geography of warfare, urban growth (almost all Italian cities were built near a saltworks) and most of the world's trade routes. Kurlansky even links the 'whimsical, non-geometric' pattern of North America's secondary roads to salt: 'The roads are simply widened footpaths and trails... originally cut by animals looking for salt.' These are the salt-lines of history, invisible on a map but brought beautifully to light by Kurlansky.

The book is broadly chronological. We begin at a saltworks in ancient China and end nearly 500 pages later amid the health wars over salt in the twenty-first century. Along the way, the reader is rewarded with superb thumbnail histories of the world's main civilisations; digressions into ketchup, chilli pepper, olives, embalming techniques, pickling and mustard gas; dozens of salty recipes; a beautiful little essay on Matisse, fauvism and anchovy fishermen.

Kurlansky is especially good on the metaphysics of salt, its metaphoric connotations and its religious significances. He draws our attention to the unrecognised ways in which salt has crystallised into our language. Salad is so named because the Romans liked to salt their vegetables. Salacious is from the Latin salax, meaning a man in love: literally, 'in the salted state'. The Roman army paid its soldiers in salt: thus the word salary and, indeed, soldier. And thus 'to be worth your salt', to earn your pay.

If there is a downside to Salt, it's that it lacks the bite of Cod. Cod was the product of a highly sensitive ecological imagination. Infused with a sadness for the passing of the cod, that book was a heartfelt elegy. Kurlansky's latest lacks this unifying attitude and occasionally lapses into a recital of statistics and factoids. Salt is at its very best when it is peppery: there is, for instance, a brilliant and acrimonious chapter on the British salt laws in India and Gandhi's now famous salt march to the Gujarat coast.

This is several books in one: a food history, a recipe book, a travelogue and a cultural history. It contains images which will abide with you: the body of a Bavarian salt miner prised from collapsed salt caves centuries after his death, for instance, perfectly preserved right down to the bright colours of his clothes. It is also stylishly written and wonderfully learned, covering a vast geographical and historical acreage. William Blake famously suggested that the world was to be seen in a grain of sand; Kurlansky has seen it in a grain of salt.